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AC and DC capitalization

  1. Feb 12, 2014 #1
    I was a little surprised to find out that the IEEE recommends using "ac" and "dc" for alternating/direct current rather than "AC" and "DC." (Google "Recommended Unit Symbols, SI Prefixes, and Abbreviations - IEEE")

    Apparently non-capitalized is the "correct" way but I've mostly seen them capitalized. Capitalized seems to make more sense since they're acronyms... Does anyone know why they're not supposed to be capitalized?

    Also, my main question: are "ac" and "dc" really preferred in cases where I can choose either way? (I'm not submitting to IEEE) From my experience, most people seem to use the upper-case acronyms, and I suspect most people will be thrown off by the lower-case acronyms even if they're technically correct.
  2. jcsd
  3. Feb 12, 2014 #2


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    I agree w/ you that lower case will likely be off-putting to most people. Even laymen are used to seeing those terms and seeing them capitalized. I can't imagine why the IEEE would even SUGGEST using lower case, much less actually recommend it.
  4. Feb 12, 2014 #3


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    I usually capitalize them when using them alone, to make them more obvious visually. When appending them to modify a quantity, I usually use lower case, like with Vdc, Vac, Vrms. Of course, I don't set the standards... :smile:
  5. Feb 12, 2014 #4


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    I agree w/ that but as attachments (subsidiarly elements) in a term, that's a different story than when they are stand-alone.
  6. Feb 12, 2014 #5
    There's a sort of convention that ac stuff is designated with lower case, dc stuff with upper case, like: 208v 3-phase power; 12V car battery. I don't think it's exactly cast in concrete or anything. And the IEEE can recommend anything they want, it doesn't necessarily mean everyone will abide by it.
  7. Feb 12, 2014 #6
    I've seen that convention used for variables, (e.g. i(t) and v(t) for time-varying current and voltage versus I and V for constant current and voltage) but I've never seen it used for units...

    I know I'm not bound by IEEE recommendations, but I'm confused as to why they would so specifically go against the obvious choice (capitalization) if there's no good reason for it. Even their document says that "ac" and "dc" are an exception compared to other acronyms, but they don't explain why. If there is indeed a good reason then I'd probably prefer to follow the convention, but I can't seem to find what that reason is.

    Also, I noticed that they recommend "var" for reactive power rather than "VAR" or "VAr," which seems inconsistent with using "VA" for real power... Again, I'm just confused why they go against what seems like the obvious choice to me.
  8. Feb 13, 2014 #7
    I think the idea is that AC and DC are becoming more like nouns rather than an acronym. It's probably like using laser instead of LASER. LASER actually stands for "light amplification by stimulated emission of radiation".

    It still doesn't make sense to use "ac" because I would read that as something like "ack" every time I saw it. I couldn't even pronounce "dc" but my Polish friends probably could. Maybe there are Slavic language speaking people taking over the IEEE? I'm kidding of course.
  9. Feb 13, 2014 #8
    A bit of googling seems to confirm what you're saying; however, I agree that it still doesn't make much sense. I would think the fact that we say "ay see" rather than "ack" would solidify ac as an acronym rather than a word.
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